In a class of electronic control systems it is often necessary to determine how long of a time that a system's main power was off. In particular, electronic control systems, used in automotive engine control applications, need to know how long an engine was off. In an engine control this is important because the engine can function differently dependent on how long a time that the engine was inactive. In particular, given stricter emissions regulations, a fuel control strategy executing in the engine control is preferably modified dependent on an estimated amount of fuel fumes remaining in the engine after the engine is inactive. The amount of fuel fumes remaining in the engine after the engine is inactive can be estimated by knowing how long of a time that the engine was off, or inactive. Typically, this inactive time is measured when the engine's ignition keyswitch is off and is referred to as a key-off time.
Since an engine can only run with main power applied to the engine control through the ignition keyswitch, knowing how long of a time that the system's main power was off or inactive will be indicative of how long a time that the engine was inactive or off.
Once the system's main power inactive time is determined, the fuel control strategy can be altered dependent on this key-off time. A typical strategy would be to add less fuel to a starting sequence if the engine was only off for a short time. This action will prevent emission of excess unburned fuel during the engine starting sequence.
In an automotive operating environment, the ignition keyswitch key-off time must be measured accurately over a several hour period. This must be done while imposing a very small current drain from a vehicle's battery so as not to drain it excessively. At the same time the solution must be mechanically robust to survive the extreme range of operating temperatures, shock, and vibration characteristic of an automotive operating environment.
In general, prior art schemes can be categorized into two approaches. A first approach relies on a relatively low current drain time keeping approach using a low frequency time base circuit comprised of a crystal resonator driving a binary counter. While the ignition keyswitch is off the crystal resonator and counter are powered by a keep-alive power source. During the time the ignition keyswitch is off, the binary counter accumulates transitions of a signal provided by the crystal resonator. When the ignition keyswitch is turned on the counter value is interpreted by the engine control's microcontroller and the key-off time is determined. A problem with this approach is that the crystal resonator is a relatively fragile device. Because of this it is difficult to reliably mount the crystal resonator in an engine control module.
A second prior art scheme relies on a relatively high current drain but more accurate time-keeping scheme. In this scheme a high-frequency quartz crystal replaces the crystal resonator. Although this approach has sufficient accuracy it also is difficult to reliably mount the quartz crystal in an engine control module. Furthermore, this approach requires much more current drain from the keep-alive power source making it an unattractive solution.
Additionally, both approaches are relatively expensive and require a significant amount of physical space in engine control modules that are required to be physically smaller and smaller.
What is needed is an improved apparatus and method that is more physically compact, reliable, economical, and is more manufacturable.